The drilling of wells in the recovery of oil and gas is typically done with a hollow drill string. As the well is being drilled, drilling fluids are pumped down the bore of the string. The drilling fluid passes through openings in the drill bit and returns to the surface through the annulus surrounding the string, carrying the cuttings produced by the drill bit. The drilling fluids can then be recycled to remove the cuttings so that they may be used again.
Traditional methods of recycling drilling fluid include using a centrifuge to separate the liquid from the cuttings. In large drilling operations, to keep up with the volume of drilling fluid used, it is necessary to use either a very large centrifuge or to use a multitude of centrifuges. In either case, the costs of operating such a drilling fluid recycling system are substantial.
It is known to use a settling tank in combination with a centrifuge in a fluid recycling system. The settling tank is used as a preliminary step to settle the cuttings from the fluid. The cuttings often remain in suspension in the fluid and are often referred to as “solids”. Flocculating agents may be introduced into the tank to assist in the settling of solids. The drilling fluids are pumped into the receiving end of the tank. A typical settling tank may have a plurality of transverse walls or baffles that form a plurality of compartments within the tank. Each wall can have an opening to permit the flow of fluid from an upstream compartment to a downstream compartment. As fluid flows from compartment to compartment, solids in the fluid settle to the bottom of the tank.
Once fluid reaches the collecting end of the tank, it is withdrawn from the tank to be used in the drilling operation. The settled solids are conveyed towards the receiving end of the tank using an auger. A slurry of settled solids and fluid are withdrawn from the tank and pumped through a centrifuge. Fluid recovered from the centrifuge is re-introduced into the tank at the receiving end.
While using the combination of settling tank and centrifuge is an improvement in comparison to using a centrifuge by itself in practice, this circuit is often unable to keep up with the throughput of drilling fluid required in drilling a well. It is often necessary to temporarily halt drilling of the well until the settling tank and centrifuge can catch up and recover enough drilling fluid to commence drilling operations again.
It is, therefore, desirable to provide a settling tank that overcomes the shortcomings of the prior art and provides a method and apparatus that is capable of recycling drilling fluid in sufficient quantity for typical drilling operations.